This invention relates to a method of treating cancer. More specifically, this invention relates to a mechanism by which tolerance of the body to malignant tissues is reduced using an immunosuppressant molecule, unacylated A-protein.
A-protein is a cellular enzyme that was first isolated from vertebrate rod photoreceptor cells in 1982 (Schmidt et al. (1983) Invest. Ophthalmol. Vis. Sci. 24:244). It has also been described in scientific literature under the names GP (Cockcroft (1987) Trends Biochem. Sci. 12:75-78), recoverin (Dizhoor et al. (1992) J. Biol. Chem. 267:16033-16036) and CAR protein (Thirkill et al. (1993) Arch. Ophthalmol. 111:974-978). A-protein has been characterized as a GTP-binding protein (g-protein) (Schmidt et al. (1987) Invest. Ophthalmol. Vis. Sci. 28:94) that regulates phosphinositide metabolism by activating phospholipase C (Schmidt et al. (1988) Invest Ophthalmol. Vis. Sci. 29:123).
A-protein exists in two forms (Schmidt et al. (1989) Invest. Ophthalmol. Vis. Sci. 30:172; and Dizhoor et al. (1992) J. Biol. Chem. 267:16033-16036); as a monomer of 23,000 daltons which is soluble in the cytosol, and as a co-synthetically modified form to which a fatty acid is attached by the action of the enzyme N-myristoyl transferase (NMT; E.C.2.3.1.97) (Dizhoor et al. (1992) J. Biol. Chem. 267:16033-16036). The modified form of A-protein tends to self-associate as stable pentameric homopolymers with an approximate molecular weight of 120,000 daltons which are peripherally bound on the inner aspect of the cell membrane.
In its peripherally membrane-bound form, A-protein is activated by a growth-factor receptor imbedded in the plasmalemma subsequent to activation of the receptor by a growth factor. The activation of this metabolic cascade mechanism results in a sustained release of calcium into the cytosol which ultimately stimulates the cell to divide. This general scheme is referred to as signal transduction (see U.S. Pat. No. 5,100,661).
In non-ocular tissues, A-protein, by definition, transduces growth signals (Whitman et al. (1986) in Phosphoinositides and Receptor Mechanisms (Alan R. Liss, Inc.) pp. 197-217). The protein is expressed in mitotically active cells including malignant tissues. A-protein is expressed inside affected malignant cells and into the blood stream (Thirkill et al. (1993) Arch. Ophthalmol. 111:974-978). Fragments of the protein are also displayed on the surface of malignant cells (Thirkill et al. (1992) Invest. Ophthalmol. Vis. Sci. 33:2768-2772).
Gery et al. (Invest. Ophthalmol. Vis. Sci. (1994) 35:3342-3345) have demonstrated that A-protein is highly immunoreactive in a dose-dependent fashion in its myristoylated form when used to immunize rats. In contrast, the unmodified form of A-protein was ineffective in inducing antibody production in mice. In addition, booster immunization with the unmodified form of the protein could significantly diminish the titer of antibodies produced by mice immunized with the myristoylated form of A-protein.
It has been appreciated for some time that the immune system is a very specific mechanism for excision of neoplastic tissues (Abbas et al. in Cellular and Molecular Immunology (B. Saunders Co., Philadelphia, Pa.) (1991) pp. 335-352). However, the lack of immunogenicity of tumor cells has presented a stumbling block to the successful reactivation of the antitumor mechanisms of the immune system.
Thus, what is needed are methods of treating cancer which cause the immune system to regain the ability to recognize neoplastic tissue as an antigenic target and which recruit a directed autoimmune response to the tumor, thereby causing its regression.